In the lack of something else do on Christmas or New Year's Eve, "bed IT IS".

IT IS by itself, used at the end of a sentence, is not an idiom. It's a phrase that, when used at the end of a sentence implies an action that's going to be taken.

Let's say you are on a soup diet. Then, you can say: For dinner, soup it is. ( For dinner, soup as usual.)

Note: you can't contract IT IS when used at the very end of a sentence because it is there for emphasis, and if contracted it would lose its emphatic meaning, thus creating a grammatical error in that particular situation.